1. Field of the Invention
The present invention relates generally to electromagnetic interference (EMI) shielding, and, more particularly, to a new and improved shielding vent for a shielded enclosure.
2. Description of the Prior Art
Diagnostic equipment, such as magnetic resonance imaging equipment, are typically used within a shielded enclosure. The shielded enclosure is used to inhibit electromagnetic radiation, such as radio frequency radiation and the like, from interfering with the operation of the equipment in the enclosure. Such enclosures may be a room or some similar structure that is located within a parent or larger room or structure. The shielded enclosure may include a single shield or multiple shields of electrically conductive metal and/or metal mesh or screen that provides the necessary shielding of the space within the enclosure. In certain instances, the shield is in the form of a conductive metal skin or layer, such as a thin sheet or foil of copper, that is disposed on the outer walls of the enclosure.
The shielded enclosure and in certain instances, equipment located in the enclosure, need to be provided with adequate venting. The vents that are used for this purpose must still provide electromagnetic interference (EMI) shielding so that even though fluid may be allowed to flow through the vent, the vent nevertheless still will sufficiently attenuate any electromagnetic radiation that might adversely affect the equipment being used in the enclosure. This attenuation needs to be effective with respect to any waves that have a frequency below a given cutoff frequency. In order to provide this necessary electric field shielding, vents for such shielded enclosures have used honeycomb structures or panels disposed within the vent.
U.S. Pat. Nos. 3,546,359 (Ciccarelli), 3,553,343 (Garlington), 3,580,981 (Lamp et al.), 3,584,134 (Nichols et al.) disclose shielded vents that can be used as vents or ventilating units for shielded or protected enclosures. In each of these patents, the structure that is used to provide the vent with RFI or EMI protection is a honeycomb panel or structure. For example, back-to-back honeycomb panels 26 and 28 are used in a vent 10 disclosed in the Ciccarelli '359 patent. The panels 26 and 28 are inclined upwardly and inwardly toward the junction between the panels. In the Garlington '343 patent, a honeycomb grill 15 (shown particularly in FIG. 1 of that patent) is disposed in an air duct housing 12 having a right angle bend. The Lamp et al. '981 patent also discloses the use of a honeycomb 12 in a ventilating panel 10 to provide EMI shielding. Similarly, the Nichols et al. '134 patent discloses the use of honeycomb elements 15a and 15b to provide shielding in a vent for a shielded room. While each of these patents disclose structures for shielded vents, the disclosed vents utilize a honeycomb structure or panel to provide the necessary EMI shielding.
In the case of shielded enclosures or rooms designed for diagnostic equipment, such as magnetic resonance medical equipment, the vents used in these enclosures must not only provide the necessary EMI shielding, but also must be capable of venting or relieving a large amount of air in a short period of time. This is because such diagnostic equipment typically uses a superconducting, supercooled electromagnet. The magnet is cooled in a cryogenic system using liquid gas, such as liquid nitrogen or liquid helium, maintained at a temperature near absolute zero. Under certain circumstances, the liquid gas may be quenched to room temperature. In such instances, the liquid gas is rapidly warmed to room temperature in a matter of a few seconds. This rapid warming of the liquid gas results in a rapid increase in the volume of the gas within the cryogenic system which must be vented to the outside of the enclosure. Unless the gas is so vented, the electromagnet used in the diagnostic equipment can be damaged.
The use of superconducting, supercooled electromagnets in such diagnostic equipment necessitates the use of a vent that is capable of venting significant volumes of air within a short period of time when the electromagnets are quenched from a relatively cold temperature to room temperature. Unfortunately, honeycomb structures, such as the ones used in the vents disclosed in the above discussed patents, present small cross sectional flow areas that tend to freeze up and inhibit the ability of the vent to relieve large volumes of air from within such shielded rooms in a short period of time.
In order to provide the necessary cross section fluid flow areas that are needed to vent a sudden increase in air pressure that might develop in such systems, vent pipes having a substantial diameter and no honeycomb structures have been used. However, these types of pipes need to be of a substantial length in order to provide effective EMI shielding. In fact, the length of the pipe may need to be as much as four times greater than the diameter of the pipe in order for such a vent to provide a desirable amount of EMI shielding. For example, a vent pipe having an eight inch diameter is used with some shielded enclosures. Such a pipe would have to be as much as thirty two inches in length to provide a desired amount of EMI shielding. However, a vent pipe having such a substantial length tends to present physical problems in that it may interfere with other equipment, ducts or lines that are used in or around the enclosure.